This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to self-sealing hydraulic systems for use in process transmitters.
Process transmitters are used to monitor process variables, such as pressure, temperature, flow and level of process fluids used in a variety of industrial processes. For example, process transmitters are widely used in the chemical manufacturing and oil refining industries to measure variables of process fluids situated throughout production facilities. Process transmitters are typically employed throughout manufacturing or processing facilities at multiple locations to monitor a variety of process variables.
Process transmitters include sensors that produce an electrical output in response to physical changes in the process variable. For example, capacitive pressure transducers or piezoresistive pressure transducers produce an electrical output as a function of the pressure of a process fluid. Typically, the pressure of the process fluid is transmitted to the sensor through a closed hydraulic system that is in contact with the process fluid at one end and the sensor at another end. As the pressure is sensed, the electrical output of the sensor is processed by the transmitter's circuitry so it can be monitored as an indication of the process variable magnitude.
The accuracy of the process transmitter depends on the ability of the closed hydraulic system to convey the magnitude of the process fluid pressure to the sensor. Typically, the hydraulic system is comprised of a hydraulic passageway that is filled with a precise level of fill fluid. At a first end of the hydraulic passageway is an isolation diaphragm that separates the fill fluid from the process fluid. At a second end of the hydraulic passageway is a sensor diaphragm that links the sensor with the fill fluid. The fill fluid typically comprises a hydraulic fluid that conveys the process fluid pressure from the isolation diaphragm to the sensor diaphragm. In other embodiments, remote seals serve as extensions of the passageway and comprise a capillary tube having additional fill fluid. As the process fluid pressure fluctuates, the process fluid exerts a corresponding force on the isolation diaphragm at the first end of the hydraulic system. The fill fluid present in the hydraulic system transmits the force to the first end of the hydraulic passageway to the isolation diaphragm. At a second end of the hydraulic passageway is a sensor diaphragm that links the sensor with the fill fluid. The force deflects the sensor diaphragm, thereby causing the sensor to alter its electrical output. Thus, the electrical output is directly related to the hydraulic nature of the fill fluid. The accuracy of the process transmitter output is related to the quantity and quality of fill fluid in the passageway and remote seal of the hydraulic system. Small leaks can reduce the quantity of fill fluid, which can introduce several inaccuracies such as diaphragms bottoming out. Leaks also allow air to enter the system which can introduce inaccuracies because gas is compressible.
Due to harsh usage, corrosion, material variations, manufacturing variations, etc., the hydraulic systems of process transmitters can develop microscopic leaks. Over time these microscopic leaks can lead to reduced accuracy performance. Since the leaks are insubstantial, there is little physical evidence of the leaking fill fluid. There is also little initial evidence in the decline in performance of the process transmitter from such small leaks. Until a substantial amount of fill fluid has leaked from the hydraulic system such that the deficiency in the process transmitter's performance is readily apparent, inaccuracies in performance can go undetected. If not discovered, the microscopic leaks will ultimately lead to a complete inability of the hydraulic system to transmit the magnitude of the process variable to the sensor.